Raman shifting is generally done by focusing a pump laser beam in a cell containing a high pressure gas or a liquid. A hollow tube with reflecting walls and a diameter of the same order as that of the focused laser beam may be inserted, in some cases, in the raman cell which tube serves to contain the pump beam and lengthen the gain zone. High repetition rate operation may require some means of gas recirculation for some raman media in order to minimize optical distortions caused by heat absorbed in the raman conversion process.
The main limitation of present pumping method is that when one uses a high enough pump power density to ensure efficient raman conversion, the raman medium suffers from what is known as optical breakdown. This will result in the raman gain going to zero with lasing or amplification stopping immediately once such a breakdown occurs. A second, particularly serious, limitation when one uses a narrow line-width pump laser is the onset of Brillouin backscattering which efficiently reflects the pump laser beam back to the source and also prevents further raman conversion. Precautions must be taken against this second limitation in order to protect the laser pump source from damage due to the reflected radiation, such as the use of expensive Faraday rotators. Another limitation in liquids is the self-focusing of the pump beam in a liquid raman medium. This phenomenon has prevented the use of many liquids for efficient raman conversion since the self-focusing can seriously damage optical components and even break the windows of the cell containing the raman liquid.